Soft contact lenses offer a viable alternative to spectacles for the correction of visual defects such as myopia, hyperopia and astigmatism.
Manufacturing of contact lenses has been the subject of numerous studies and patent applications over the last thirty years. Substantial effort has been directed towards developing new manufacturing technologies which can be fast, reliable, industrially applicable and substantially reduce lens cost.
One process which holds considerable promise to achieve these objectives is direct injection moulding of thermoplastic polymers. Thermoplastic materials which are suitable for contact lens manufacture are known (see for instance WO 2011/055110 and WO 2011/055108hereby incorporated by reference). These poly (ethylene glycol) based polymers exhibit a number of superior properties such as increased oxygen permeability and resistance to lens dehydration both of which can improve the safety and comfort of a lens.
Injection moulding is used to manufacture a wide range of medical devices such as urological, cardiovascular and thoracic drain catheters. These products are formed from large moulds and commonly require secondary finishing in order to make the device suitable for use.
Articles of manufacture formed from current injection moulding apparatus and techniques commonly comprise a witness mark or sprawl. This may take the form of additional material protruding out of the article of manufacture.
In contrast, contact lenses are very thin parts (typical around 100 μm thin) and require an optical centre and a precision edge which plays a key role in determining the comfort of a lens. Due to both comfort, safety and optical concerns, witness marks or sprawls are clearly not acceptable for contact lenses. Additional material protruding into the eye will greatly increase the discomfort and irritability associated with the contact lens, as well as increasing the risk of scratching of the outside surface of the eye.
In the past it has proved difficult to mold contact lenses having the required physical dimensions due to the relatively small size, and the accuracy necessary to obtain the required optical properties.
The Centre Thickness of a Contact lens in the zero gel state is circa 90 μm. It has previously been found that in order to directly injection mould into a cavity area this thin would require pressures & speeds that exceed the capability of a standard injection moulding machine. Using standard injection pressures and speeds would result in voids, flow marks and surface defects that would render the lens unsuitable for on eye use.
There is a need therefore for a fast, reliable direct injection moulding apparatus and method capable of forming these parts as well as providing a significant reduction of lens manufacture cost of goods compared to existing multi-step manufacturing step based processes which are currently used to form lenses.
The present invention provides an apparatus suitable for the manufacture of articles of manufacture such as contact lenses.